1. Field of the Invention
The invention relates to a method of manufacturing and of increasing the selectivity of an integral asymmetric membrane for separating gases from each other.
2. Technology Review
Membrane processes can be operated with low energy consumption in comparison to conventional gas separation methods such as low temperature distillation, and chemical and physical absorption. Generally, the process of separating a gas mixture is carried out isothermally, i.e. without phase change. Membrane processes are distinguished by high process flexibility, simple operation, and low maintenance costs.
The chief cost factor in a membrane process is the choice of a suitable membrane material, in addition to the employment of suitable modular systems and appropriate process operation and control. An "ideal" membrane for gas separation should have the following properties:
1. Very high selectivities with regard to various separation tasks (e.g., He/N.sub.2, H.sub.2 /N.sub.2, CO.sub.2 /CH.sub.4, and other separations).
2. Highly permeable gas flow (flux) (e.g., He, H.sub.2, and CO.sub.2).
3. Minimum plastification (i.e., becoming plastic) of the membrane.
4. High heat stability.
5. High mechanical strength.
6. High chemical resistance, particularly against "impurities" in a natural gas mixture (water vapor and H.sub.2 S in biogases and geogases).
7. Good reproducibility of membrane manufacture.
8. Manufacturable by simple, easy, and maximally economical method.
For a long time there has been a lack of success in manufacturing an "ideal" membrane, because some of the requirements are in mutual opposition. In general, highly permeable membranes (e.g., silicones) are not very selective, and highly selective membranes, typically glass-like polymers, are not very permeable.
In order to be able to compete with commercially available membranes, a newly developed membrane must have a selectivity of .alpha..sub.j.sup.i =20. Moreover, for economical application of membranes to gas separation, a selectivity of .alpha..sub.j.sup.i .gtoreq.40 should be attained, with correspondingly high permeabilities to rapidly permeating components (see Wensley, G. C., Jakabhazy, S. Z., "High performance gas separation membranes", paper presented at Am. Inst. Chem. Engrs. 1984 Winter National Meeting, Atlanta, Ga., March 11-14, 1984).
To achieve high permeabilities it is essential that the separating layer of the membranes be kept as small as possible, because the gas flow is inversely proportional to the membrane thickness. Accordingly, a number of methods have been developed for producing a membrane which is as thin as possible. The manufacture of homogeneous, symmetric polymer films is simple to achieve: A thin polymer solution is applied to a suitable support by pour-coating or film-drawing. After evaporation of the solvent (which may be readily or difficultly volatizable), a film of polymer forms on the support. However, homogeneous, symmetrical membranes with film thickness .ltoreq.5 micron are difficult to handle, and usually have defects which limit their application in gas separations. Membranes fabricated from glass-like polymers must have film thickness .ltoreq.1 micron in order to achieve economical permeabilities (at least 0.05 cu m per sq m per hr per bar). It is possible to manufacture homogeneous, symmetrical polymer films of this order of magnitude only with very costly techniques, because impurities which are present (e.g., dust particles) with sizes of 3000 Angstrom are unavoidable in membrane fabrication (for example, see U.S. Pat. No. 4,230,463). This particle size is adequate to initiate defect loci in the membrane, resulting in sharply lower separation performance (lower selectivity).
A currently prevalent method of manufacturing membranes is the phase inversion process, which leads to integral asymmetric membranes. An integral asymmetric membrane is comprised of a thin selective layer (about 0.2-1 micron) and a porous base layer, which base layer provides the membrane with strength and durability. Important advantages of this method are the relatively simple manufacturing technique and the possibility of fabricating membranes with different structures and therefore different separation characteristics.
The greatest difficulty with integral asymmetric membranes is the same as with homogeneous, symmetrical membranes--the problem of forming an absolutely defect-free, selective film. The only commercially employed integral asymmetric phase-inversion membrane which is usable for gas separation (because it has a defect-free, selective film) is a cellulose acetate membrane originally developed for sea water desalination (Envirogenics, Separex). Although the cellulose acetate membranes provide good separating characteristics, particularly for CO.sub.2 /CH.sub.4 separation, their use for gas separation is characterized by the following drawbacks:
1. Sensitivity to condensed water (irreversible breakdown);
2. Sensitivity to microbiological attack;
3. Plastification of the cellulose acetate membranes, particularly during CO.sub.2 separation;
4. Low heat resistance (up to about 70.degree. C.); and
5. Relatively high manufacturing cost, because cellulose acetate membranes cannot be directly air-dried. (If direct air drying is employed, the porous base layer collapses.) A change of solvents is required in order to accomplish drying.
The enumerated drawbacks of cellulose acetate membranes tend to substantially detract from the separating characteristics. In view of these problems, there has been a great deal of research aimed at fabricating integral asymmetric membranes from other polymers. It is not possible to fabricate integral asymmetric membranes from any polymer with good material-specific properties. Thus, e.g., Kapton-brand polyimide film by E. I. duPont de Nemours & Co. should be usable in particular to separate CO.sub.2 from CH.sub.4 (Chern, R. T., Koros, W. J., Hopfenberg, H. B., Stannett, V. T., 1985, "Material selection for membrane-based gas separations", in "Material science of synthetic membranes", ACS Symposium Ser. Vol. 269, Ed. Douglas R. Lloyd, Amer. Chem. Soc., Wash., DC), due to polyimides's very high selectivity, very high heat resistance, and mechanioal strength and durability. Because polyimide is practically insoluble, however, phase inversion membranes cannot be produced from it using the usual techniques.
There are other suitable polymers from which gas separation membranes can be fabricated, namely polysulfones and soluble polyimides. These polymers are not commercially available, however. They have not been manufactured on a scale greater than laboratory scale, even though they have improved chemical resistance, heat resistance, and mechanical strength and durability, because they do not offer substantially better separation characteristics, particularly for CO.sub.2 /CH.sub.4 separation. The separation characteristics of various integral asymmetric phase-inversion membranes are presented in Tables 1 and 2.
TABLE 1 ______________________________________ Permeability of integral asymmetric phase- inversion membranes to various gases, for gas separations at 22.degree. C.: Cellulose acetate Polysulfone** Polyimide Gas (m.sup.3 /m.sup.2 hbar) (m.sup.3 /m.sup.2 hbar) (m.sup.3 m.sup.2 hbar) ______________________________________ He 0.80 0.50 0.51 H.sub.2 0.64 0.39 0.27 CO.sub.2 0.23* 0.20* 0.05* CH.sub.4 0.0084 0.0080 0.0015 N.sub.2 0.0084 0.0053 0.0018 ______________________________________ *Values interpolated from zero pressure. **Product being developed by Envirogenics not commercially available.
TABLE 2 ______________________________________ Ideal selectivities of integral asymmetric phase-inversion membranes: Ideal Selec- tivity Cellulose acetate Polysulfone** Polyimide ______________________________________ .alpha.*CO.sub.2 /CH.sub.4 27 25 33 .alpha.*CO.sub.2 /N.sub.2 27 38 28 .alpha.*He/N.sub.2 95 94 284 ______________________________________ .alpha.* = Ideal selectivity.
Envirogenics has stated that as of March 1984 there was no commercially available plastic which achieved an ideal selectivity of 40 for the CO.sub.2 /CH.sub.4 separation and was suitable for producing integral asymmetric membranes.
An unfilled need of the prior art is to manufacture an integral asymmetric membrane for gas separation which is particularly suitable for CO.sub.2 /CH.sub.4 separation, which membrane has improved insensitivity to water and relatively high strength and durability, as well as having high separation throughput and selectivity (permeability .gtoreq.0.1 cu m/sq m/hr/bar for CO.sub.2, and CO.sub.2 /CH.sub.4 selectivity about 30-40), which high separation throughput and selectivity are necessary for economical separation of materials.